Processes for making synthetic polymeric membranes, including hollow fiber gas separation membranes, are well documented in the art. Separation/permeation characteristics of membranes are optimized with respect to intended end use. Consequently, consideration must be given to materials and manufacturing methods to be employed in membrane manufacturing.
A superior membrane must have a good balance of chemical, mechanical, and separation characteristics in order to function properly. It is often not possible, however, to obtain high performance in all aspects of membrane properties from a single material. Thus, a method to decouple the membrane separation/permeation characteristics from the bulk mechanical properties is frequently needed.
One method extensively employed in the art to decouple mechanical properties from membrane separation characteristics is via the composite membrane approach, wherein a thin separation layer is deposited by a solution coating method on a preformed substrate. Preparation of such membranes is described in U.S. Pat. Nos. 4,243,701; 4,826,599 and 4,840,819.
Another method that accomplishes this goal is coextrusion. Two distinctly different polymer solutions are coextruded simultaneously to form a bilayer hollow fiber. The technique permits the active sheath layer to be formed from a polymer with superior separation/permeation characteristics while the core layer that makes up the majority of membrane mass is formed from a common polymer with good mechanical and thermal characteristics. Examples of this method are taught by Ekiner et al. in U.S. Pat. No. 5,085,676 and by Kusuki et al. in Japanese Patent Application No. Sho 62-253785.
Yet another method used to optimize membrane properties employs casting solutions containing blends of two or more polymers. There are numerous examples in the art of fluid separation membranes advantageously prepared from blends of polymers. Kraus et al. in U.S. Pat. No. 5,076,935 teach the use of polyethersulfone/phenoxy resin blends to make porous isotropic membranes. Nunes et al. describe preparation of asymmetric membranes useful for ultrafiltration from blends of polyvinylidene fluoride and poly(methyl methacrylate) in the Journal of Membrane Science, 73(1992), 25-35. The practice of blending polymers also has been used effectively in the preparation of gas separation membranes. Yamada et al. in U.S. Pat. No. 4,832,713 disclose fabrication of gas separation membranes from blends of polyetherimide mixed with materials such as polycarbonates or polysulfones. Kohn et al. in U.S. Pat. No. 5,055,116 utilize miscible blends of polyimides of specific chemical compositions to prepare gas separation membranes. Burgoyne, Jr. et al. in U.S. Pat. No. 5,061,298 also describe preparation of membranes from specific polyimide blends.
Each of the aforementioned methods of membrane preparation, however, has some limitations. Coextrusion is a relatively complicated process because of the need for costly, specialized hardware such as dual-annulus spinnerettes and the need to formulate two spinning dopes in order to prepare a hollow fiber membrane. Preparation of composite membranes by solution coating methods is a two-step process wherein selection of the coating separation material is frequently limited by the solvent resistance characteristics of the substrate. Polymer blending techniques have been optimized around polymers with specific chemical structures. Polymers frequently have to be thermodynamically compatible, i.e. miscible, in order to form superior membranes. Thus there still exists a need for a simple, efficient method of manufacturing membranes with improved combination of mechanical and separation properties.